Preparation Method of Super Absorbent Polymer

This application claims the benefits of Korean Patent Applications No. 10-2022-0176831 filed on Dec. 16, 2022, No. 10-2022-0177310 filed on Dec. 16, 2022 and No. 10-2023-0181009 filed on Dec. 13, 2023 with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

The present disclosure relates to a preparation method of a super absorbent polymer. More specifically, it relates to a preparation method of a super absorbent polymer exhibiting excellent absorption properties with significantly reduced generation of fine particles.

A super absorbent polymer (SAP) is a type of synthetic polymeric material capable of absorbing 500 to 1000 times its own weight of moisture. Various manufacturers have denominated it with different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), and the like. Such super absorbent polymers started to be practically applied in sanitary products, and they are now being widely used for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultices, or the like.

These super absorbent polymers have been widely used in the field of hygienic materials such as diapers or sanitary napkins. In such hygienic materials, the super absorbent polymer is generally contained in a state of being spread in the pulp. In recent years, however, continuous efforts have been made to provide hygienic materials such as diapers having a thinner thickness. As a part of such efforts, the development of so-called pulpless diapers and the like in which the pulp content is reduced or pulp is not used at all is being actively advanced.

In the case of the above-described hygienic materials in which the pulp content is reduced or pulp is not used, the super absorbent polymer is included in a relatively high ratio, so that the super absorbent polymer particles are inevitably included in multiple layers in the hygienic materials. In order for the entire super absorbent polymer particles included in multiple layers to more efficiently absorb a large amount of liquid such as urine, the super absorbent polymer basically needs to exhibit excellent absorption performance as well as a high absorption rate.

Meanwhile, such a super absorbent polymer is generally prepared by performing the step of polymerizing monomers to prepare a hydrogel polymer containing a large amount of moisture; and the step of drying the hydrogel polymer, and then pulverizing it into polymer particles having a desired particle diameter. However, when the hydrogel polymer is dried and then pulverized as described above, a large amount of fine powder is generated, which has a problem of lowering the physical properties, especially absorption properties, of the super absorbent polymer to be finally prepared.

Accordingly, in order to reuse the fine powder, it is common to mix the fine powder with water for agglomeration to produce a fine reassembly, and then add the fine reassembly to the process such as drying/pulverization/classification. However, the water used herein may cause problems such as increased energy usage during drying and increased load on the device, which may reduce productivity of super absorbent polymer production.

Meanwhile, the most common method to increase the absorption rate is to expand the surface area of the super absorbent polymer by forming a porous structure in the super absorbent polymer. In order to increase the surface area of the super absorbent polymer, a method of forming a porous structure in the base resin powder by including a foaming agent in the monomer composition and performing cross-linking polymerization is generally selected.

However, the use of a foaming agent has the disadvantage of lowering physical properties of the super absorbent polymer, such as surface tension, permeability, or bulk density, and increasing the generation of fine particles. Accordingly, the development of technology that can improve the absorption rate of the super absorbent polymer without the use of a foaming agent is continuously requested.

Accordingly, there is a continuous demand for preparing a super absorbent polymer without generating fine particles in order to fundamentally solve these problems.

Accordingly, there is provided a preparation method of a super absorbent polymer exhibiting excellent absorption properties while significantly improving the absorption rate and significantly reducing the generation of fine particles during the process by forming micropores in the polymer through a micronization process to increase the surface area.

In order to solve the above problems, there is provided a preparation method of a super absorbent polymer including the steps of:

According to the preparation method of a super absorbent polymer of the present disclosure, it is possible to provide a super absorbent polymer exhibiting excellent absorption properties while significantly improving the absorption rate and significantly reducing the generation of fine particles during the process by forming micropores in the polymer through a micronization process to increase the surface area.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “include”, “have”, or “possess” when used in this specification, specify the presence of stated features, steps, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, steps, components, or combinations thereof.

As the present invention can be variously modified and have various forms, specific embodiments thereof are shown by way of examples and will be described in detail. However, it is not intended to limit the present invention to the particular form disclosed and it should be understood that the present invention includes all modifications, equivalents, and replacements within the idea and technical scope of the present invention.

As the present invention can be variously modified and have various forms, specific embodiments thereof are shown by way of examples and will be described in detail. However, it is not intended to limit the present invention to the particular form disclosed and it should be understood that the present invention includes all modifications, equivalents, and replacements within the idea and technical scope of the present invention.

Hereinafter, the preparation method for a super absorbent polymer and the super absorbent polymer of the present disclosure will be described in more detail.

The terminology used herein is only intended to refer to specific embodiments and is not intended to limit the present invention. Further, as used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite.

According to one embodiment of the present disclosure, there is provided a preparation method of a super absorbent polymer including the steps of:

The terminology “polymer” in the present disclosure is in a state in which water-soluble ethylene-based unsaturated monomers are polymerized, and may include all moisture content ranges, or all particle diameter ranges.

In addition, the terminology “super absorbent polymer” is used to encompass all of a cross-linked polymer or a base resin in the form of powder consisting of super absorbent polymer particles in which the cross-linked polymer is pulverized, and the cross-linked polymer or the base resin further processed, for example, drying, pulverization, classification, surface cross-linking, etc., to be in a state suitable for commercialization, depending on the context.

In addition, the terminology “fine powder” or “fine particle” refers to particles having a particle diameter of less than 150 μm among the super absorbent polymer particles. The particle diameter of these polymer particles may be measured according to the EDANA (European Disposables and Nonwovens Association) WSP 220.3.

In addition, the terminology “chopping” refers to cutting the hydrogel polymer into small pieces of millimeters in order to increase drying efficiency, and is used separately from pulverization to a level of micrometers or normal particles.

In addition, the terminology “micronizing (micronization)” refers to pulverizing the hydrogel polymer to a particle diameter of several tens to hundreds of micrometers, and is used separately from “chopping”.

The hydrogel polymer obtained by polymerization of acrylic acid-based monomers undergoes processes such as drying, pulverization, classification, and surface cross-linking, and is sold as a super absorbent polymer in the form of powder. Recently, attempts have been made continuously to provide a super absorbent polymer exhibiting a more improved absorption rate.

The most common method to increase the absorption rate is to increase the surface area of the super absorbent polymer by forming a porous structure in the super absorbent polymer. In order to increase the surface area of the super absorbent polymer, a method of forming a porous structure in the base resin powder by including a foaming agent in the monomer composition, and then performing cross-linking polymerization is generally selected.

However, the use of a foaming agent has the disadvantage of lowering physical properties of the super absorbent polymer, such as surface tension, permeability, or bulk density, and increasing the generation of fine particles. Accordingly, the development of technology that can improve the absorption rate of the super absorbent polymer without the use of a foaming agent is continuously requested.

Meanwhile, super absorbent polymers are conventionally prepared by performing cross-linking polymerization on a water-soluble ethylene-based unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal cross-linking agent and a polymerization initiator to form a hydrogel polymer, and then drying the formed hydrogel polymer, followed by pulverization to a desired particle size. At this time, a chopping process of cutting the hydrogel polymer into particles of several millimeters is usually performed before the drying process to facilitate drying of the hydrogel polymer and increase the efficiency of the pulverization process. However, due to the stickiness of the hydrogel polymer in this chopping process, the hydrogel polymer cannot be pulverized to micro-sized particles and becomes an aggregated gel. When the hydrogel polymer in the form of an aggregated gel is dried, a plate-shaped dried product is formed. In order to pulverize it to micro-sized particles, a multi-stage pulverization process is required, so there has been a problem in that many fine particles are generated in this process.

Specifically, the super absorbent polymer has conventionally been prepared by including the following steps.

As described above, the chopped hydrogel polymer has an aggregated gel shape with a size of about 1 cm to 10 cm. Such a chopped hydrogel polymer is laminated on a belt whose bottom is made of a perforated plate, and dried by hot air supplied from the bottom or top. Since the polymer dried by the above drying method has a plate shape rather than a particle shape, the classification step after pulverization has been performed by coarse pulverization, then classification, followed by fine pulverization again, then classification so that the particles to be prepared become normal particles, that is, particles having a particle diameter of 150 μm to 850 μm. Since the amount of the fine particles separated in the final classification step is about 20 wt % to about 30 wt % based on the total weight of the final super absorbent polymer, the separated fine particles were mixed with an appropriate amount of water, reassembled, and then introduced in the chopping step or before drying for reuse.

However, problems such as causing an increase in equipment load and/or energy consumption have occurred when a fine reassembly mixed with water is re-injected into the pulverization or drying process for reuse of the fine particles, and the remaining fine particles that have not been classified caused deterioration of physical properties of the super absorbent polymer.

Accordingly, the present inventors have found that the generation of fine particles in the conventional preparation method is greatly affected by a pulverization process, and the generation of fine particles in the preparation process can be significantly reduced by controlling the pulverizing conditions of the polymer.

Meanwhile, the most common method to increase the absorption rate is to increase the surface area of the super absorbent polymer by forming a porous structure in the super absorbent polymer. In order to increase the surface area of the super absorbent polymer, a method of forming a porous structure in the base resin powder by including a foaming agent in the monomer composition and then performing cross-linking polymerization is generally selected.

However, the use of a foaming agent has the disadvantage of lowering physical properties of the super absorbent polymer, such as surface tension, permeability, or bulk density, and increasing the generation of fine particles due to the pores formed by the foaming agent, which sharpen the shape of the particle surface. Accordingly, the development of technology that can improve the absorption rate of the super absorbent polymer without the use of a foaming agent is continuously requested.

As a result of repeated research to solve this problem, it has been confirmed that when polymerization is first performed in a state where the acidic groups are not neutralized to form a polymer, the hydrogel polymer is micronized by applying high intensity mechanical shear force and then the acidic groups of the polymer are neutralized; when the acidic groups of the polymer are neutralized to form a hydrogel polymer, and then the hydrogel polymer is micronized; or when the acidic groups present in the polymer are neutralized at the same time as micronization, agglomerated hydrogel particles with micropores could be formed. As the hydrogel polymer prepared through this process is manufactured in the form of particles with stable micropores of 100 μm or less, and the pulverization and drying processes proceed under milder conditions, the generation of fine particles during the processes can be significantly reduced. In addition, it has been confirmed that the micronization process using the high-intensity mechanical shear force can significantly improve the absorption rate by forming micropores in the hydrogel polymer without using a separate foaming agent in the polymerization step.

Meanwhile, the micronization process of the hydrogel polymer may preferably be performed in the presence of a surfactant. When using a surfactant in the micronization step, agglomeration of particles can be effectively controlled, and thus productivity can be further improved by lowering the load on the machine.

In addition, when polymerization is first performed in an unneutralized state to form a polymer, and then acidic groups present in the polymer are neutralized, a longer chain polymer can be formed and cross-linking is incomplete, so that it is possible to reduce the content of extractable components present in a non-crosslinked state.

The extractable components have a property of being easily eluted when the super absorbent polymer comes into contact with a liquid. When the content of the extractable components is high, most of the eluted extractable components remain on the surface of the super absorbent polymer and make the super absorbent polymer sticky, thereby reducing permeability. Therefore, it is important to keep the content of extractable components low in terms of permeability.

According to one embodiment of the present disclosure, as the polymerization is performed in an unneutralized state, the content of extractable components is lowered, and thus permeability of the super absorbent polymer can be improved.

Further, the super absorbent polymer prepared according to one embodiment of the present disclosure may have a uniform particle size distribution, and thus it is possible to provide a super absorbent polymer excellent in absorption properties such as water retention capacity and absorbency under pressure, rewet characteristics, and absorption rate.

Hereinafter, the preparation method of a super absorbent polymer of one embodiment will be described in more detail for each step.

First, polymerization is performed on a monomer composition containing a water-soluble ethylene-based unsaturated monomer having acidic groups, an internal cross-linking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylene-based unsaturated monomer having acidic groups and the internal cross-linking agent are cross-link polymerized.

The above step may include a step of preparing a monomer composition by mixing the water-soluble ethylene-based unsaturated monomer having acidic groups, an internal cross-linking agent, and a polymerization initiator, and a step of polymerizing the monomer composition to form a polymer.

The water-soluble ethylene-based unsaturated monomer may be any monomer commonly used in the preparation of a super absorbent polymer. Specifically, the water-soluble ethylene-based unsaturated monomer may be a compound represented by the following Chemical Formula 1:

Preferably, the monomer may be at least one selected from the group consisting of (meth)acrylic acid, and a monovalent (alkali)metal salt, a divalent metal salt, an ammonium salt and an organic amine salt of the acid.

When (meth)acrylic acid and/or a salt thereof is used as a water-soluble ethylene-based unsaturated monomer, it is advantageous to obtain a super absorbent polymer having improved absorption performance. In addition, maleic anhydride, fumalic acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamide-2-methyl propanesulfonic acid, (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethyleneglycol (meth)acrylate, polyethyleneglycol (meth)acrylate, (N,N)-dimethylaminoethyl (meth)acrylate, (N,N)-dimethylaminopropyl (meth)acrylamide, and the like may be used as the monomer.

Herein, the water-soluble ethylene-based unsaturated monomer has acidic groups. As described above, a polymer was formed by cross-linking polymerization of a monomer in which at least some of the acidic groups are neutralized by a neutralizing agent. Specifically, in the step of mixing the water-soluble ethylene-based unsaturated monomer having an acidic group, an internal cross-linking agent, a polymerization initiator, and a neutralizing agent, at least some of the acidic groups of the water-soluble ethylene-based unsaturated monomer were neutralized.

However, according to one embodiment of the present disclosure, polymerization is first performed in a state where acidic groups of the water-soluble ethylene-based unsaturated monomer are not neutralized to form a polymer.

The water-soluble ethylene-based unsaturated monomers (e.g., acrylic acid) in a state where acidic groups are not neutralized are in a liquid state at room temperature, and have high miscibility with a solvent (water), and thus exist in the monomer composition in the form of a mixed solution. However, the water-soluble ethylene-based unsaturated monomers in which the acidic groups are neutralized are in a solid state at room temperature, have different solubility depending on the temperature of the solvent (water), and has lower solubility at a lower temperature.